This application claims the benefit of European Application Serial Number 02 005 435.9 filed on Mar. 8, 2002.
The invention relates generally to wire bonding, and specifically to a method and apparatus for the production and quality control testing of an ultrasonically bonded wire connection.
Bonding is an extremely widely used method of connecting wires in electronic devices of all kinds, in order to form contacts with electronic components and in particular integrated circuits (chips). The quality of these connections determines to a considerable extent the performance and reliability of the relevant electronic devices. Therefore the manufacturers of these devices pay great attention to monitoring the quality of the connections, and the manufacturers of wire-bonding machines are faced with a demand for ever more reliable testing and process-control systems.
One of the furthest developed and most widespread bonding methods is ultrasonic (xe2x80x9cwedgexe2x80x9d) bonding, which basically represents a micro-version of friction welding. Herexe2x80x94as described, for example, in the applicant""s patent U.S. Pat. No. 4,619,397xe2x80x94an aluminium wire in contact with a substrate surface, to which it is to be connected in such a way that the two materials remain in contact, is put into rapid oscillation by an ultrasound transducer and simultaneously pressed against the surface. Under the influence of the compression (bonding) force and the oscillation (bonding) power, an oxide coating on the surface is broken up and, as severe deformation and local heating occur, the wire and surface materials intermingle to produce bonding at the interface between wire and surface.
A more detailed description of this method is unnecessary here, as it has long been well known to those skilled in the art.
For testing bonded connections produced in this way, many methods have been established, of which reference is made here only to that disclosed in the applicant""s patent U.S. Pat. No. 4,984,730. This document describes a test method based on monitoring a deformation of the wire during the bonding process and comparison with a standard or reference curve. When the measured deformation curve is too far from the reference curve, this is regarded as indicating inadequate quality of the bonded connection, and when such unacceptable deviations are detected, the procedure should be interrupted and the relevant process parameters readjusted.
A method of testing connections produced by ultrasonic bonding that is similar, in a certain sense, is described in the patent DE 44 47 073 C1. Here the firmness of the connection is the decisive parameter for the bond quality. It is proposed that the relevant parameter to be monitored is the velocity or time course of deformation of the wire to be bonded and the time course of the amplitude of the bonding tool (wedge amplitude) during the bonding process; the results of this monitoring are to be compared with reference data. This method enables the firmness of each individual connection to be tested with no substantial extra expenditure of time.
In the applicant""s patent U.S. Pat. No. 5,314,105 a system is proposed for controlling an ultrasonic wire-bonding process in real time, or quasi-real time, by monitoring the time dependence of deformation of the bonded wire. Specifically, it is proposed that the power input to the ultrasound transducer should be kept at a high level until a marked increase in deformation appears in the time-dependence curve, at which point the power is reduced to a predetermined lower level. Furthermorexe2x80x94in a preferred procedurexe2x80x94it is proposed to turn the ultrasound transducer off completely when the wire deformation has substantially reached a predetermined final value.
In the applicant""s patent GB 2 271 305 B a procedure for controlling a soldering or bonding process of the reflow type is proposed, in which the power applied to drive prespecified sections of the process is adjusted to different levels. In this document the application of this principle for a wire bonder of the kind cited above is also mentioned.
The previously proposed methods and apparatus have proved not to provide optimal results in all of the constellations of materials and processes encountered in practice. It has been impossible to control or regulate the bonding process in the initial phase (the first five to ten ms), because during this period the wire to be bonded was not deformed sufficiently to permit evaluation.
Hence it is the objective of the invention to provide an improved method of this generic kind as well as an apparatus suitable for implementing the method, which enable a further improvement in ensuring the quality of bonded wire connections.
This objective is achieved with respect to the method by a method having the characteristics given in claims 1 and 2, and with respect to the apparatus by an apparatus having the characteristics given in claim 10.
The invention includes the fundamental idea of monitoring, by means of a suitable parameter, a reaction of the bonding process upon the toolxe2x80x94the ultrasound transducerxe2x80x94and thereby detecting a substantial change of state of the work piece, namely the wire-substrate unit during formation of the bonded connection. It further includes the idea of utilizing as this parameter the impedance measured at the transducer. Finally, the invention includes the idea of comparing the recorded time dependence of the representative parameter (impedance) with a suitable reference, which is obtained during the manufacture of qualitatively perfect bonded connections.
The invention enables an improved control or regulation of the bonding process, in particular in its initial phase, because a quantity which has an informative time-dependence during this phase is used as an input variable for the control or regulation process.
In a first, relatively independent implementation of the idea behind the invention, the proposed method is a manufacturing procedure insofar as it comprises, in addition to the above-mentioned aspects, the concept of controlling the power input to the ultrasound transducer (the bond power) or the force that the bonding tool exerts on the wire (bond weight), or both of these variables.
In a second implementation of the idea behind the invention, the proposed method represents a test procedure for quality control of the bonded connection that is produced, insofar as it allows the decision as to quality of the bonded connection to be derived from the recorded time dependence of the impedance in comparison to the reference curve.
The inventor has established, on the basis of extensive experimental investigations, that a particular characteristic shape of the impedance-time curve, distinguished by a relatively steep rise to a clear maximum and a subsequent decline, initially steeper and then becoming less steep, can be taken as an unambiguous indicator of a qualitatively high-value connection. It will be evident that for different bonding-process parameters, in particular for wires differing in composition and dimensions as well as substrate surfaces of various kinds, the specific time dependence of the impedance and in particular the position of the maximum and the rise and fall gradients are different. However, for each constellation of process parameters suitable reference curves can be constructed with a limited number of experiments, which can be used for comparison with a time-dependence curve obtained for the ongoing process.
Evaluation of the curve shape for controlling or regulating the bonding process is preferably carried out by comparison of the overall time dependence recorded while the bonded connection is being produced, or else only a characteristic section of the overall curve, with a reference curve or a set of reference curves. A significant assessment of the quality of the bonded connection is provided, as far as can be seen at present, by evaluation including three predetermined impedance-time reference surfaces (in the following termed xe2x80x9creference windowsxe2x80x9d), through which the recorded curve representing the time dependence of impedance must pass in order to justify the conclusion that the bonded connection is of high quality.
Alternative possibilities that can be considered in principle include a numerical evaluation of characteristic curve sectionsxe2x80x94for example, to determine the peak impedance value and a slope of the falling flank in a region of the curve next to the maximumxe2x80x94and numerical comparison with corresponding values obtained from reference measurements. It will be evident that with such a method of evaluation, as well, certain tolerance values with respect to the reference curves should be specified in order for the bonded connection currently being produced to qualify as xe2x80x9cgoodxe2x80x9d.
In an especially preferred embodiment of the proposed production and/or testing method, the proposed evaluation of the impedance-time curve is combined with an evaluation of the time dependence of the deformation of the wire being bonded (which as such is fundamentally known from the patent U.S. Pat. No. 5,314,105). In particular, the previously known idea of a steplike reduction of the bond power at the time when there is a distinct increase in wire deformation is combined with the idea in accordance with the invention that the bond power and/or bond weight is/are to be controlled in dependence on the characteristic curve for time dependence of the transducer impedance. In analogy, from the combined evaluation of the two time dependencesxe2x80x94each in comparison to a reference curve obtained from bonded connections known to be of good qualityxe2x80x94an especially reliable decision can be made regarding the quality of the bonded connection.
As also happens when the quality test or process control is based exclusively on the impedance-time dependence, here the quality test or process control on the basis of a combined evaluation of the impedance-time curve and deformation-time curve is in practice preferably automatic, for which purpose at least components of a regulation sequence or system are provided. This regulation component has a special character in the case of the combined method, because the impedance detected at the transducer and the bond power fed into the transducer are originally physically related to one another, and this relationship is in a certain sense xe2x80x9cmodulatedxe2x80x9d during the process of producing the bonded connection. This relationship provides a physical basis for the said regulation component.
Aspects of the apparatus for implementing the method correspond to the above-mentioned procedural steps and aspects in a way readily discernible by a person skilled in the art, so that it is not necessary to discuss in detail all the components of the apparatus. However, it should be pointed out that an arrangement provided for quality testing comprises, according to the above description, a decision-making device connected to the evaluation device, to compare the impedance-time curve with suitable references and report a good/bad decision. In an arrangement provided for bonding-process control, on the other hand, the evaluation unit is connected to a bond-power control unit and/or bond-weight control unit, which it controls according to a prescribed control algorithm (which has been derived from experimental findings).